The following Patents and published patent applications are believed to represent the current state of the art:
U.S. Pat. No. 4,986,275;U.S. Pat. No. 5,143,063;U.S. Pat. No. 5,143,073;U.S. Pat. No. 5,209,221;U.S. Pat. No. 5,301,660;U.S. Pat. No. 5,431,621;U.S. Pat. No. 5,507,790;U.S. Pat. No. 5,526,815;U.S. Pat. No. 5,884,631;U.S. Pat. No. 6,039,048;U.S. Pat. No. 6,071,239;U.S. Pat. No. 6,113,558;U.S. Pat. No. 6,206,873;GB2303552;WO02/24076JP58181399;JP11187493;JP10277483;JP8089893;